Horsehair worms don’t need to be eliminated. They are completely harmless to people, pets, livestock, and plants. They cannot infect or parasitize any vertebrate animal. If you’ve found one in a water trough, swimming pool, birdbath, or garden puddle, you can simply pick it up and toss it somewhere else, or leave it alone entirely. That said, finding a long, dark, writhing worm is understandably unsettling, and there are practical steps you can take to stop seeing them around your home.
What You’re Actually Looking At
Horsehair worms are typically 30 to 40 centimeters long (roughly a foot), though some reach 120 centimeters (about 4 feet). They’re extremely thin, often just 1 millimeter in diameter, giving them a thread-like or hair-like appearance. The color ranges from mauve-brown to black, and they have a featureless body with a blunt head and slightly swollen tail. When found in water, they often coil and twist slowly, which can look alarming.
People frequently mistake them for parasitic roundworms or other harmful organisms. The key difference is size and proportion: horsehair worms are dramatically longer and thinner than any parasite that could infect a human or pet. If you’ve found a very long, very skinny worm in standing water or crawling on wet pavement after rain, it’s almost certainly a horsehair worm.
Why They’re in Your Water
Horsehair worms spend their juvenile life as parasites inside insects, particularly crickets, beetles, and grasshoppers. The cycle works like this: tiny larvae hatch in water and get swallowed by an insect that comes to drink. Inside the insect, the worm grows to full size over weeks or months. When mature, the worm manipulates its host to seek out water, where it bursts free. The adult worm then lives in the water briefly to mate and lay eggs.
This is why you find them in water troughs, pet bowls, swimming pools, birdbaths, puddles, and even toilets. An infected cricket or beetle found its way to that water source, and the worm emerged. The adult worms don’t feed at all. They exist only to reproduce and then die. Their free-living phase is short.
They Cannot Harm You or Your Animals
There is no evidence that horsehair worms parasitize humans. Researchers reviewing decades of reported human encounters concluded that, with the exception of a few unexplained historical cases, no true parasitism occurs. The association between horsehair worms and humans is accidental. If a person or animal accidentally swallows one in drinking water, the worst outcome is mild, temporary intestinal discomfort. The worm cannot establish an infection, reproduce, or survive inside a vertebrate body.
The same applies to dogs, cats, horses, cattle, and birds. Horsehair worms parasitize only invertebrates. Your livestock can drink from a trough that contains horsehair worms without any health risk. You don’t need to treat the water or replace it on their account.
How to Remove Them
If you simply don’t want to look at them, the removal process is straightforward:
- Pick them out. Use your hand, a stick, or a net. Drop the worm in a garden bed or grassy area away from the water source. It will die on its own once it dries out.
- Dump and refill standing water. Empty birdbaths, pet bowls, and small containers, rinse them, and refill. This removes any worms and eggs.
- Clean water troughs regularly. A periodic scrub and water change keeps troughs free of eggs, though again, the worms pose zero risk to animals drinking from them.
- Reduce standing water around your property. Puddles, clogged gutters, and poorly drained low spots all create habitat where adult worms mate and lay eggs, and where infected insects are drawn to release their parasites.
Preventing Future Appearances
Since horsehair worms depend entirely on insect hosts, reducing the insect population around water sources is the most effective long-term strategy. Crickets and beetles are the primary hosts. Outdoor lighting near pools, troughs, and birdbaths attracts these insects at night, so moving lights away from water features or switching to yellow bulbs (which attract fewer insects) can help. Sealing cracks in your home’s foundation and around doors keeps crickets from entering, which prevents the occasional startling discovery of a horsehair worm in a sink or toilet after a cricket dies indoors near plumbing.
Keep grass trimmed and remove debris like leaf piles, woodpiles, and ground cover near water sources. These are hiding spots for crickets and ground beetles. If you have a serious cricket problem around your home, addressing that with bait or perimeter treatments will indirectly reduce horsehair worm sightings.
Covering water troughs and pools when not in use prevents insects from reaching the water in the first place. Fine mesh screens over rain barrels and livestock tanks are a simple, chemical-free barrier.
Why Pesticides Don’t Make Sense Here
There are no pesticides labeled for horsehair worms, and none are needed. Applying chemicals to water sources to kill a harmless organism creates far more risk than the worm itself. Horsehair worms are actually beneficial in a small way: they help control cricket and beetle populations naturally. Each worm that completes its life cycle means one fewer adult insect. If you’re finding horsehair worms regularly, it means your local cricket population is healthy enough to sustain them, and the worms are doing some of that population control for you.
Iowa State University Extension’s guidance on horsehair worms is direct: no control measures are needed. The University of California’s Integrated Pest Management program echoes this. The consensus across entomology programs is that horsehair worms are a curiosity, not a problem.